Slow sculpting on high density meshes
but you are referring to lag when nagivating or when sculpting? dyntpo its better for having less polycount when sculpting but it affects the mesh and without a way to to delete it or undo what you done after finished the sculpt and also it works with triangules instead or quads. multires instead save this information in the modifier and makes a even subdivision in the whole mesh, but some times not necesary to have all the mesh subvided in areas that you wont sculpt, and it stays the original polycount and topology of the original low poly base mesh used. but you can use both like he does https://www.youtube.com/watch?v=Nc3cRVG3me4 also avoid to have matcaps and the outline select option checked because it will put it more slower when its very high poly.